| Network Working Group | I. Liusvaara |
| Internet-Draft | Independent |
| Intended status: Standards Track | December 9, 2015 |
| Expires: June 11, 2016 |
CFRG curves and signatures in JOSE
draft-liusvaara-jose-cfrg-curves-00
This document defines how to use curves and algorithms from IRTF CFRG elliptic curves work (Diffie-Hellman and signatures) in JOSE.
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Internet Research Task Force (IRTF) Crypto Forum Research Group (CFRG) selected new elliptic curves and signature algorithms for asymmetric key cryptography. This document defines how those curves and algorithms are to be used in JOSE in interoperable manner.
This extends [RFC7517] and [RFC7518]
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].
All inputs to and outputs from the the ECDH and signature functions are defined to be octet strings, with the exception of output of verfication function, which is a boolean.
A new key type (kty) value "OKP" (Octet Key Pair) is defined for public key algorithms that use octet strings as private and public keys. It has the following parameters:
Note: Do not assume that there is an underlying elliptic curve, despite the existence of the "crv" and "x" parameters.
When calculating thumbprints [RFC7638], the three public key fields are included in the hash. That is, in lexographic order: "crv", "kty" and "x".
The following signature algorithms are defined here (to be applied as values of "alg" parameter). All these have keys with subtype of the same name:
alg value: subtype: The algorithm: Ed25519 Ed25519 Ed25519 Ed25519ph Ed25519ph Ed25519ph Ed448 Ed448 Ed448 Ed448ph Ed448ph Ed448ph
The key type for these keys is "OKP" and key subtype for these algorithms MUST be the same as the algorithm name.
The keys of these subtypes MUST NOT be used for ECDH-ES.
[TBD: Merge the alg values into a single one that can perform signing with any signature-capable OKP subtype? That would remove a source of possible errors, since then the message and key could not mismatch in algorithm.]
Signing for these is preformed by applying the signing algorithm defined in [I-D.irtf-cfrg-eddsa] to the private key (as private key), public key (as public key) and the JWS Signing Input (as message). The resulting signature is the JWS Signature value. All inputs and outputs are octet strings.
Verification is performed by applying the verification algorithm defined in [I-D.irtf-cfrg-eddsa] to the public key (as public key), the JWS Signing Input (as message) and the JWS Signature value (as signature). All inputs are octet strings. If the algorithm accepts, the signature is valid, otherwise it is invalid.
The following key subtypes defined here for purpose of ECDH-ES:
subtype: ECDH Function: X25519 X25519 X448 X448
The key type used with these keys is "OKP". These subtypes MUST NOT be used for signing.
The "x" parameter of "epk" field is set as follows:
Apply the appropriate ECDH function to the ephemeral private key (as scalar input) and the standard basepoint (as u-coordinate input). The output is the value for "x" parameter of "epk" field. All inputs and outputs are octet strings.
The Z value (raw key agreement output) for key agreement is determined as follows:
Apply the appropriate ECDH function to the ephemeral private key (as scalar input) and receiver public key (as u-coordinate input). The output is the Z value. All inputs and outputs are octet strings.
Security considerations from [I-D.irtf-cfrg-curves] and [I-D.irtf-cfrg-eddsa] apply here.
Some algorithms interact in bad ways (e.g. "Ed25519" and "Ed25519ph"). For this reason, those algorithms have different subtypes, so keys for each are not mixed up.
Do not separate key material from information what key algorithm group it is for. When using keys, check that the algorithm is compatible with the key algorithm group for the key. To do otherwise opens system up to attacks via mixing up algorithms. It is practicularly dangerous to mix up signature and MAC algorithms.
Do not assume that signature also binds the key used for signing, it does not (there are also other widespread signature algorithms where this binding fails, as such binding is not part of the definition of secure signature primitive). As an example of such failure, the Ed25519ph signature of X under key (Ed25519ph,Y) is identical to Ed25519 signature of SHA512(X) under key (Ed25519,Y). And often it takes only setting a few bits of message (easy to do by brute force) to make the message valid enough to be processed in some very surprising way.
If key generation or batch signature verification is performed, a well-seed cryptographical random number generator is REQUIRED. Signing and non-batch signature verification are deterministic operations and do not need random numbers of any kind.
Mike Jones for comments on initial pre-draft.
The following is added to JSON Web Key Types Registry:
The following is added to JSON Web Key Parameters Registry:
The following is added to JSON Web Signature and Encryption Algorithms Registry:
The following is added to JSON Web Key Elliptic Curve Registry:
| [RFC2119] | Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997. |
| [RFC4648] | Josefsson, S., "The Base16, Base32, and Base64 Data Encodings", RFC 4648, DOI 10.17487/RFC4648, October 2006. |
| [I-D.irtf-cfrg-curves] | Langley, A. and M. Hamburg, "Elliptic Curves for Security", Internet-Draft draft-irtf-cfrg-curves-11, October 2015. |
| [I-D.irtf-cfrg-eddsa] | Josefsson, S. and I. Liusvaara, "Edwards-curve Digital Signature Algorithm (EdDSA)", Internet-Draft draft-irtf-cfrg-eddsa-01, December 2015. |
| [RFC7517] | Jones, M., "JSON Web Key (JWK)", RFC 7517, DOI 10.17487/RFC7517, May 2015. |
| [RFC7518] | Jones, M., "JSON Web Algorithms (JWA)", RFC 7518, DOI 10.17487/RFC7518, May 2015. |
| [RFC7638] | Jones, M. and N. Sakimura, "JSON Web Key (JWK) Thumbprint", RFC 7638, DOI 10.17487/RFC7638, September 2015. |
To the extent possible, the examples use material lifted from test vectors of [I-D.irtf-cfrg-curves] and [I-D.irtf-cfrg-eddsa]
{"kty":"OKP","crv":"Ed25519",
"d":"nWGxne_9WmC6hEr0kuwsxERJxWl7MmkZcDusAxyuf2A"
"x":"11qYAYKxCrfVS_7TyWQHOg7hcvPapiMlrwIaaPcHURo"}
The hexadecimal dump of private key is:
9d 61 b1 9d ef fd 5a 60 ba 84 4a f4 92 ec 2c c4 44 49 c5 69 7b 32 69 19 70 3b ac 03 1c ae 7f 60
And of the public key:
d7 5a 98 01 82 b1 0a b7 d5 4b fe d3 c9 64 07 3a 0e e1 72 f3 da a6 23 25 af 02 1a 68 f7 07 51 1a
This is the public parts of the previous private key (just omits "d"):
{"kty":"OKP","crv":"Ed25519",
"x":"11qYAYKxCrfVS_7TyWQHOg7hcvPapiMlrwIaaPcHURo"}
The JWK thumbprint canonicalization of the two above examples is (linebreak inserted for formatting reasons)
{"crv":"Ed25519","kty":"OKP","x":"11qYAYKxCrfVS_7TyWQHOg7hcvPapiMlrwI
aaPcHURo"}
Which has the SHA-256 hash of: 90facafea9b1556698540f70c0117a22ea37bd5cf3ed3c47093c1707282b4b89
The JWS protected header is:
{"alg":"Ed25519"}
This has base64url encoding of:
eyJhbGciOiJFZDI1NTE5In0
The payload is (text):
Example of Ed25519 signing
This has base64url encoding of:
RXhhbXBsZSBvZiBFZDI1NTE5IHNpZ25pbmc
The JWS signing input is (concatenation of base64url encoding of the (protected) header, a dot and base64url encoding of the payload) is:
eyJhbGciOiJFZDI1NTE5In0.RXhhbXBsZSBvZiBFZDI1NTE5IHNpZ25pbmc
Applying Ed25519 signing algorithm to the private key, public key and the JWS signing input yields signature (hex):
53 18 48 60 b1 c6 83 7f 4d 54 22 e9 40 05 43 fd 47 1f 3a 69 c6 48 2c cb 15 9a 17 62 42 e2 21 b1 5c 72 63 9b fe a3 9b b2 08 f3 2c ab 1f 27 0f b8 36 57 1c 52 0b d8 ac 41 eb 45 b3 55 d0 77 19 01
Converting this to base64url yields:
UxhIYLHGg39NVCLpQAVD_UcfOmnGSCzLFZoXYkLiIbFccmOb_qObsgjzLKsfJw-4NlccU gvYrEHrRbNV0HcZAQ
So the compact serialization of JWS is (concatenation of signing input, a dot and base64url encoding of the signature:
eyJhbGciOiJFZDI1NTE5In0.RXhhbXBsZSBvZiBFZDI1NTE5IHNpZ25pbmc.UxhIYLHGg 39NVCLpQAVD_UcfOmnGSCzLFZoXYkLiIbFccmOb_qObsgjzLKsfJw-4NlccUgvYrEHrRb NV0HcZAQ
The JWS from above example is:
eyJhbGciOiJFZDI1NTE5In0.RXhhbXBsZSBvZiBFZDI1NTE5IHNpZ25pbmc.UxhIYLHGg 39NVCLpQAVD_UcfOmnGSCzLFZoXYkLiIbFccmOb_qObsgjzLKsfJw-4NlccUgvYrEHrRb NV0HcZAQ
This has 2 dots in it, so it might be valid JWS. Base64url decoding the protected header yields:
{"alg":"Ed25519"}
So this is Ed25519 signature. Now the key has: "kty":"OKP" and "crv":"Ed25519", so the key is valid for the algorithm (if it had other values, the validation would have failed).
The signing input is the part before second dot:
eyJhbGciOiJFZDI1NTE5In0.RXhhbXBsZSBvZiBFZDI1NTE5IHNpZ25pbmc
Applying Ed25519 verification algorithm to the public key, JWS signing input and the signature yields true. So the signature is valid. The message is base64 decoding of the part between the dots:
Example of Ed25519 signing
The public key to encrypt to is:
{"kty":"OKP","crv":"X25519","kid":"Bob"
"x":"3p7bfXt9wbTTW2HC7OQ1Nz-DQ8hbeGdNrfx-FG-IK08"}
The public key from target key is (hex):
de 9e db 7d 7b 7d c1 b4 d3 5b 61 c2 ec e4 35 37 3f 83 43 c8 5b 78 67 4d ad fc 7e 14 6f 88 2b 4f
The ephemeral secret happens to be (hex):
77 07 6d 0a 73 18 a5 7d 3c 16 c1 72 51 b2 66 45 df 4c 2f 87 eb c0 99 2a b1 77 fb a5 1d b9 2c 2a
So the ephemeral public key is X25519(ephkey,G) (hex):
85 20 f0 09 89 30 a7 54 74 8b 7d dc b4 3e f7 5a 0d bf 3a 0d 26 38 1a f4 eb a4 a9 8e aa 9b 4e 6a
This is packed into ephemeral public key value:
{"kty":"OKP","crv":"X25519",
"x":"hSDwCYkwp1R0i33ctD73Wg2_Og0mOBr066SpjqqbTmo"}
So the protected header could for example be:
{"alg":"ECDH-ES+A128KW","epk":{"kty":"OKP","crv":"X25519",
"x":"hSDwCYkwp1R0i33ctD73Wg2_Og0mOBr066SpjqqbTmo"},
"enc":"A128GCM","kid":"Bob"}
And sender computes as the DH Z value as X25519(ephkey,recv_pub) (hex):
4a 5d 9d 5b a4 ce 2d e1 72 8e 3b f4 80 35 0f 25 e0 7e 21 c9 47 d1 9e 33 76 f0 9b 3c 1e 16 17 42
The receiver computes as the DH Z value as X25519(seckey,ephkey_pub) (hex):
4a 5d 9d 5b a4 ce 2d e1 72 8e 3b f4 80 35 0f 25 e0 7e 21 c9 47 d1 9e 33 76 f0 9b 3c 1e 16 17 42
Which is the same as sender's value (the both sides run this through KDF before using as AES128-KW key).
The public key to encrypt to is (linebreak inserted for formatting reasons):
{"kty":"OKP","crv":"X448","kid":"Dave"
"x":"PreoKbDNIPW8_AtZm2_sz22kYnEHvbDU80W0MCfYuXL8PjT7QjKhPKcG3LV67D2
uB73BxnvzNgk"}
The public key from target key is (hex):
3e b7 a8 29 b0 cd 20 f5 bc fc 0b 59 9b 6f ec cf 6d a4 62 71 07 bd b0 d4 f3 45 b4 30 27 d8 b9 72 fc 3e 34 fb 42 32 a1 3c a7 06 dc b5 7a ec 3d ae 07 bd c1 c6 7b f3 36 09
The ephemeral secret happens to be (hex):
9a 8f 49 25 d1 51 9f 57 75 cf 46 b0 4b 58 00 d4 ee 9e e8 ba e8 bc 55 65 d4 98 c2 8d d9 c9 ba f5 74 a9 41 97 44 89 73 91 00 63 82 a6 f1 27 ab 1d 9a c2 d8 c0 a5 98 72 6b
So the ephemeral public key is X448(ephkey,G) (hex):
9b 08 f7 cc 31 b7 e3 e6 7d 22 d5 ae a1 21 07 4a 27 3b d2 b8 3d e0 9c 63 fa a7 3d 2c 22 c5 d9 bb c8 36 64 72 41 d9 53 d4 0c 5b 12 da 88 12 0d 53 17 7f 80 e5 32 c4 1f a0
This is packed into ephemeral public key value (linebreak inserted for formatting purposes):
{"kty":"OKP","crv":"X448",
"x":"mwj3zDG34-Z9ItWuoSEHSic70rg94Jxj-qc9LCLF2bvINmRyQdlT1AxbEtqIEg1
TF3-A5TLEH6A"}
So the protected header could for example be (linebreak inserted for formatting purposes):
{"alg":"ECDH-ES+A256KW","epk":{"kty":"OKP","crv":"X448",
"x":"mwj3zDG34-Z9ItWuoSEHSic70rg94Jxj-qc9LCLF2bvINmRyQdlT1AxbEtqIEg1
TF3-A5TLEH6A"},"enc":"A256GCM","kid":"Dave"}
And sender computes as the DH Z value as X448(ephkey,recv_pub) (hex):
07 ff f4 18 1a c6 cc 95 ec 1c 16 a9 4a 0f 74 d1 2d a2 32 ce 40 a7 75 52 28 1d 28 2b b6 0c 0b 56 fd 24 64 c3 35 54 39 36 52 1c 24 40 30 85 d5 9a 44 9a 50 37 51 4a 87 9d
The receiver computes as the DH Z value as X448(seckey,ephkey_pub) (hex):
07 ff f4 18 1a c6 cc 95 ec 1c 16 a9 4a 0f 74 d1 2d a2 32 ce 40 a7 75 52 28 1d 28 2b b6 0c 0b 56 fd 24 64 c3 35 54 39 36 52 1c 24 40 30 85 d5 9a 44 9a 50 37 51 4a 87 9d
Which is the same as sender's value (the both sides run this through KDF before using as AES256-KW key).